In recent years, optical fiber communication systems capable of conducting the high-quality transmission of large-capacity information have come into practical use. As the component part of such a communication system, a directional coupling type optical device that can control optical signal at a high speed and uses optical waveguides capable of being miniaturized by integration has been developed.
FIG.1 shows the generalized composition of a conventional optical device thus developed. This optical device is composed of a first optical waveguide 11 and a second optical waveguide 12 on a LiNbO.sub.3 substrate 10. The first optical waveguide 11 and the second optical waveguide 12 have the same width, thickness and refractive index, and are disposed in parallel close to each other, thereby forming a directional coupler 13 in this parallel part. The coupling length that the movement of light between optical waveguides in the directional coupler 13 becomes 100% is defined as a perfect coupling length Lc. The directional coupler 13 is formed with a coupling length that is half of the perfect coupling length Lc. Also, a total reflection film (or total reflection plate) 14 is disposed opposed to the end faces of the first and second optical waveguides forming the directional coupler 13. Further, on the first and second optical waveguides 11, 12 forming the directional coupler 13, control electrodes 15, 16 are formed through buffer layers (not shown), and voltage can be applied to its both ends.
When voltage is not applied to the control electrodes 15, 16, the optical energy of incident light 17 supplied to the first optical waveguide 11 of this optical device gradually moves to the second optical waveguide 12 in the directional coupler 13. Then, when propagating by half of the perfect coupling length Lc to reach the total reflection film 14, half of the energy of incident light is moved to the second optical waveguide 12. At this time, the first and second optical waveguides 11, 12 have a same optical intensity, and have phases inverse to each other. It reflects totally on the total reflection film 14, then propagating through the directional coupler 13 in the reverse direction. Also in this case, the optical energy gradually moves to the second optical waveguide 12. When returning to the incidence point of the directional coupler 13, all the optical energy is moved to the second optical waveguide 12. As a result, emitting light 18 with the same optical intensity as incident light 17 is obtained from the second optical waveguide 12.
However, this optical device is composed of the first and second optical waveguides 11, 12 on the LiNbO.sub.3 substrate 10. The substrate 10 has an electro-optic effect, where the refractive index at the periphery of voltage-applied part varies by an electric field caused by the application of voltage. Thus, applying voltage to the control electrodes 15, 16, causes the refractive index of the first and second optical waveguides 11, 12 formed under the electrodes to be changed.
In this state, the coupling state between the optical waveguides changes due to the discordance of phase speed between the waveguide modes of the optical waveguides forming the directional coupler 13.
Thus, light 18 can be controlled so as not to be emitted from the second optical waveguide 12.
Namely, by applying voltage to the electrode on the optical waveguide forming the directional coupler provided on the substrate with the electro-optic effect, when supplying incident light from one optical waveguide, light can be controlled so as to be emitted from another optical waveguide or so as not to be emitted therefrom.
This means that the on/off control of emitted light can be performed.
Also, the optical device to perform such control can be miniaturized since it only has to have half of the perfect coupling length.
Such optical devices using the electro-optic effect are, for example, disclosed in Japanese patent application laid-open Nos. 63-234227(1988) and 3-256028(1991).
However, these optical devices have to use a very specific substrate with the electro-optic effect, e.g., LiNbO.sub.3 substrate. Therefore, there is a problem that especially in combining and integrating various kinds of optical devices, the manufacturing cost increases.